1. Field of the Invention
This invention relates to a thin film resistor, and more particularly to a thin film resistor which has high resistivity and has superior temperature characteristics, and which is suitable for integrated circuits.
2. Description of the Prior Art
Resistors formed of carbon, metal and oxides are known. However, since these resistors have a relatively high conductivity, they must be extremely thin in order to obtain a high resistance. Moreover, since resistors have relatively low resistances, they must be made thin and narrow and complicated manufacturing processes are required to manufacture suitable resistors.
Resistors formed of single crystal silicon are known in which a P-type semiconductive region is formed in an N-type semiconductor substrate, and an N-type semiconductive region is formed in the P-type semiconductive region. Electrodes are arranged on opposite ends of the P-type semiconductive region which serves as a resistance region. However, the resistivity of the resistance region is very low, as for example, 10.sup.2 ohm/cm.sup.2. The resistivity also changes with temperature as much as 0.5% per 1.degree. C. The temperature coefficient of the resistor is positive within the range of the practical temperatures, and thus, the resistance increases as the temperature rises. Also, the linearity of the V-I characteristic of the device deteriorates due to the formation of the depletion layer.
A resistor formed of polycrystalline silicon is also known in which polycrystalline silicon is doped with boron atoms B or phosphorous atoms P to obtain a lower resistance than plain polycrystalline silicon. It has not been known to obtain a higher resistance than plain polycrystalline silicon prior to this invention. The resistivity of pure polycrystalline silicon is about 10.sup.6 ohm cm. It is therefore necessary with a resistor of pure polycrystalline silicon to have a narrow width in the order of 1.mu.m to obtain a high resistance such as 10.sup.10 ohm/cm.sup.2. A resistor is desired which has a resistivity between 10.sup.3.sup.-4 to 10.sup.8.sup.-16 ohm cm preferably in the range between 10.sup.6 to 10.sup.12 ohm cm. The resistivity of SiO.sub.2 is about 10.sup.12.sup.-14 ohm cm. It is very difficult however, to obtain a high resistance in such range. Although it has previously been known how to lower the resistivity of polycrystalline silicon, it has not been known prior to our invention how to raise the resistivity of polycrystalline silicon.
Resistors formed of polycrystalline silicon can be used as the gate resistor for a resistance-gate type field effect transistor (SRG-FET). Boron atoms B or phosphorous atoms P can be diffused into the contact portion between a gate electrode and the gate resistor to provide good ohmic contact. However, an amplifier and a capacitor are required for applying drain signals to the gate electrode. The distortion factorfrequency characteristics of the SRG-FET can be determined by the capacitance Co of the capacitor and the resistance Rg of the gate resistor. With a time constant CoRg larger than the period 1/f of the signals frequency, the distortion factor will descrease. When the time constant CoRg is smaller than the period 1/f, the distortion factor will increase. The distortion factor can be improved by increasing the capacitance Co, however, operation of the SRG-FET is difficult because units such as an amplifier and a capacitor are required.
An SRG-FET is proposed in which a polycrystalline silicon film is formed on a SiO.sub.2 film to increase the breakdown voltage of a transistor, and in which a depletion layer is broadened by the electrical field from the polycrystalline silicon film. However, such transistor has a disadvantage in that electrical charges pass through the polycrystalline silicon film and generate a reverse leakage current.